Blast comes with freeride fin and Jag with a slalom fin.
on the Blast page you can see it's mentioned for different board sizes which fin is supplied:
BLAST LTD 100 / 115 / 130 / 145
Foil Tuttle Box / CF Spirit 34 cm, 38 cm / 42 cm / 44 cm, Black
so it should be Choco Fins Spirit (C1 ??): in custom Fanatic color
https://www.chocofins.com/range/spirit/
to buy additional fins you can check with your shop, they can order.
Fanatic actually has their own webshop for spares and fins, but I don't see currently any available.
https://shop.fanatic.com/en-gb/fins/windsurfing-fins-nbsp/